This invention generally relates to a technique for measuring of a radar signal in order to uniquely characterize, and thereby identify, the emitter. Moreover, the invention can be used to detect the presence of countermeasures employed by the radar, particularly those designed to defeat passive Doppler emitter location.
It is now typical for both time and frequency measurements to be made on a radar's signal from an observer that may be a unmanned aerial vehicle (UAV). UAV's typically have rudimentary navigation systems, so it is desirable to measure a classifying parameter that does not depend on accurate knowledge of observer kinematics or angular attitude. An apparatus and method for doing this is also valuable for use on more sophisticated high performance aircraft, since it can be used as a stand-alone system and does not require costly integration and testing for its incorporation.
Many time and frequency parameters currently used in identifying radar systems such as signal pulse width, signal RF frequency, and pulse repetition time interval make significant demands on the observer navigation systems. These parameters are all related to two parameters of particular interest for many pulse echo radars: the emitter's rest frequency .function..sub.0 and rest pulse repetition interval fundamental time difference t.sub.po. This time difference is the greatest common divisor of all the interpulse time intervals. It determines the time intervals between the pulses (300, FIG. 3). The inverse of this fundamental time difference is the pulse repetition frequency or PRF. The integer multiples of t.sub.po that form the time differences between the pulses are called pulse repetition intervals or PRI (301, 302 FIG. 3). The parameters .function..sub.0 and t.sub.po are typically unique to a radar type and thus provide an excellent means of classification. FIG. 1 shows how these parameters are generated in a typical pulse amplifier radar.
In this figure the RF and PRF frequencies are synthesized from a single reference oscillator 100 according to the method discussed by Taylor and Mattern, page 5-15, in the 1970 edition of Skolnik's Radar Handbook. As Taylor and Mattern note, although possible variations in design are almost limitless, the frequency generation method shown resembles many actual systems. This frequency generation method involves multiplying up from the reference frequency .function..sub.r 101 by integer multiples to get the coherent local oscillator frequency 111, and RF carrier frequency 121, and dividing by an integer amount to get the PRF frequency 131.
A problem in measuring .function..sub.0 and t.sub.po for the pulsed amplifier radar shown in FIG. 1 is that the reference oscillator 100 may change, either due to a repair or to a conscious attempt to prevent classification. For instance, if the crystal is replaced in the oscillator, the reference frequency .function..sub.r will change and hence all frequencies derived from it change.
Additional problems measuring these parameters arise from the motion of the observing aircraft. The observing aircraft does not measure either quantity directly, but rather their Doppler shifted values. These values are derived in many textbooks, e.g Landau and Lifshitz, The Classical Theory of Fields, 4.sup.th edition, page 117, and are, for frequency measurements ##EQU1## or, for time difference measurements ##EQU2## where .beta. is the relativistic factor ##EQU3## and
v=observer velocity PA1 v=observer speed PA1 u=emitter signal direction of arrival unit vector.
The equations for .function. and t.sub.p are often approximated by neglecting terms to second and higher order in v/c, giving for example ##EQU4## However, as will be seen, such approximations are not needed here.
Extracting the RF and PRF rest frequencies from these Doppler shifted measurements typically requires a sophisticated ESM system. For example, both precision navigation information to characterize v and
means to simultaneously measure the emitter signal direction of arrival or DOA u are needed. Such systems are heavy, costly, and are not typically
available on unmanned aircraft.
It is, therefore, an object of this invention to provide a method for measuring a classifying parameter of a radar emitter without requiring information regarding the emitter's relative bearing or other navigation data.
Another object of this invention is to provide a method for measuring a classifying parameter like the one described above which may be used to detect countermeasures employed by the radar emitter particularly designed to defeat passive Doppler emitter location techniques.